The invention relates to a traffic display for tracking aircraft; more particularly, but not exclusively, the invention relates to a display device for traffic surveillance and and collision avoidance systems in formation aircraft.
Presently, most aircraft utilize systems that provide pilots information to avoid potential collisions in the air and/or on the ground. There are many varieties of collision avoidance systems (CAS) and conflict detection systems in aircraft that fall into the following general category: (1) passive systems; and (2) active systems. Active collision avoidance systems utilize transmission broadcasts from the aircraft to determine relevant information relating to other aircraft in the area, and/or provide its own relative information to other aircraft in an area. The most prevalent active system used in the U.S. today, is the Traffic Alert and Collision Avoidance System or “TCAS.” (TCAS is internationally known as ACAS or Airborne Collision Avoidance System).
TCAS offers pilots of private, commercial and military aircraft reliable information to track traffic and avoid potential collisions with other aircraft. TCAS is a family of airborne devices that operate independently of the ground-based Air Traffic Control (ATC) systems. Since TCAS inception, three different control levels have evolved: TCAS I is intended for commuter and general aviation aircraft and provides a proximity warning only, assisting the pilot in visually acquiring intruder aircraft; TCAS II is intended for commercial airliners and business aircraft to provide pilots with traffic and resolution advisories in the vertical plane; and TCAS III, which has yet to be approved by the FAA, will purportedly provide resolution advisories in the horizontal as well as vertical plane.
TCAS detects the presence of nearby aircraft equipped with transponders that reply to ATCRBS Mode C or Mode S interrogations. When nearby aircraft are detected, TCAS tracks and continuously evaluates the potential of these aircraft to collide with its own aircraft.
For surveillance, TCAS interrogations are transmitted over an interrogation channel (e.g., 1030 MHz) from the TCAS equipped aircraft to any aircraft within the range of the transmission. The interrogation requests a reply from transponder-equipped aircraft within range of the transmission to provide their pertinent position and/or intent information. Transponder-equipped aircraft within range of the transmitted interrogation, reply over a reply channel (e.g., 1090 MHz) by providing their associated information. This information can include altitude, position, bearing, airspeed, aircraft identification and other information of the in-range aircraft to assist the TCAS in tracking and evaluating the possibilities of collision with the in-range aircraft.
Essentially, TCAS is a surveillance system and a collision avoidance system. For tracking nearby aircraft or “intruders,” a symbol depicting the surrounding aircraft is displayed on traffic displays located in the cockpit. The displayed symbols allow a pilot to maintain awareness of the number, type and position of aircraft within the vicinity of his own aircraft.
For collision avoidance, TCAS predicts the time to an intruder's closet point of approach (CPA) and a separation distance at the CPA, by calculating range, closure rate, vertical speed and altitude. TCAS provides the capability of tracking other aircraft within a certain range, evaluating collision potential, displaying/announcing traffic advisories (TAs), and depending on the type of system used (e.g., TCAS II) recommending evasive action in the vertical plane to avoid potential collisions, otherwise known as a Resolution Advisories (RAs).
It should be noted that in certain circumstances aircraft may not be detected by TCAS, for example, aircraft not equipped with operating transponders cannot reply to interrogations; military aircraft equipped with identification friend or foe (IFF) systems operating in mode 4 do not reply to interrogations; and aircraft that may not hear interrogations for one reason or another (e.g., interference, lowering landing gear when intruder was being tracked by only the bottom antenna or interference limiting).
The Federal Aviation Administrations (FAA) set guidelines for collision, warning and caution areas for implementation of TCAS II. A volume of space defines these areas, and/or a time tau (τ) to penetration of that space, around the TCAS equipped aircraft. Examples of a collision area 110, warning area 115 and caution area 150 of an aircraft 105 equipped with TCAS II, are illustrated in FIGS. 1A (top view) and 1B (perspective view). If oncoming aircraft 120 actually penetrates caution area 150 it may be designated as an intruder and a traffic advisory may be issued to the pilot or crew of TCAS equipped aircraft 105. The TA may consist of an audible warning and visual display indicating the distance and relative bearing to intruder 120. If an intruder 122 penetrates warning area 115, a resolution advisory may be issued to the crew or pilot of TCAS equipped aircraft 105. The RA may be corrective or preventive and may consist of instructions to climb or descend at a recommended vertical rate, or caution the pilot not to make changes in the present vertical rate.
The shapes, horizontal and vertical dimensions of the respective areas are a function of the range and closure rate of oncoming aircraft 120.
The time-space domain for TCAS interrogations is limited in that each interrogation-reply takes a certain period of time. When several different aircraft are interrogating in the same proximity, the amount of transponder replies can saturate the surrounding airspace and cause ATC tracking problems. To overcome this problem TCAS was designed with logic that, when a certain number of TCAS equipped aircraft are within a predetermined vicinity of each other, output power and processor interrogations are reduced. This is known as Interference Limiting. The reduction of output power effectively shortens the TCAS intruder tracking range. Low traffic density areas allow for increased transmission power whereas high traffic density areas (often called Terminal Control Areas “TCAs”) require a reduced transmission power. For example, the TCAS of an aircraft flying over Western Kansas may have a 80 nm (nautical miles) interrogation range or longer, whereas an aircraft flying near Chicago may reduce its interrogation range down to 5 nm with greater link margin. The reduction of transmission power from a low density area to a high density area may be as much as 10 dB. This is done to reduce RF interference between other TCAS equipped aircraft and to reduce RF interference with ATC ground tracking stations.
Certain aircraft, typically military aircraft, frequently fly in multi-aircraft groups known as formations. A problem occurs when all planes in a given formation are actively interrogating with their TCAS. Notably, the TCAS of planes in and outside the formation may detect a seemingly high density of planes in a traffic area due to the formation and thus reduce the transmission power of their respective broadcasts and reduce their receiver sensitivity to compensate for the perceived density. This type of unnecessary range adjustment due to reduced transmission power and reduced receiver sensitivity is referred to as “Interference Limiting” and degrades collision avoidance safety to unacceptable levels (e.g., interrogation range is significantly decreased in areas where aircraft may be flying at high speeds). Even small formations of two or three TCAS enabled aircraft may result in Interference Limiting to non-formation and formation aircraft.
Honeywell (formerly Allied Signal) developed a collision avoidance system designed to specifically address military formation-flying insufficiencies of conventional TCAS; this system is known as Enhanced TCAS or “ETCAS.” ETCAS provided means for military planes to fly in formation by offering a rendezvous-type feature in collision avoidance systems that would allow aircraft to be able to fly in a formation with other aircraft without generating RAs and TAs against one another.
However, ETCAS also generated significant Interference Limiting in non-formation aircraft. The FAA and civilian regulatory agencies of other countries severely restricted the use of TCAS, including ETCAS, during formation flying due to the resulting Interference Limiting problems. These restrictions essentially require several members in a formation to fly with their TCAS turned off, while one or a few aircraft in the formation are allowed to have their TCAS turned on. These restrictions were detrimental to the purpose of collision avoidance systems since many members of a formation have no indication of potential collision threats between themselves and non-formation aircraft as well as potential collisions threats between other members of the formation. Further, the restrictions on the use of TCAS during formation flying essentially negated any advantages of ETCAS.
The block diagram of FIG. 2 illustrates an example of Interference Limiting. As shown, a group of aircraft 210-215 are flying in formation 200 while TCAS equipped aircraft 220 is approaching formation 200. The wavy lines preceding an aircraft in FIGS. 1-3 illustrate transmission of TCAS broadcasts.
When the TCAS of aircraft 220 receives TCAS broadcasts (interrogations) from aircraft 210-214 in range of perimeter 260, and intruder tracks are formed on aircraft 210—214 within the TCAS of aircraft 220, the perceived high density of intruders 210—214 by TCAS of aircraft 220 may result in an automatic adjustment by the TCAS of aircraft 220 to a reduced surveillance range. (The reduction in the number and power of TCAS broadcasts is gradual and is not necessarily realized by a pilot or flight crew). An example of the shortened surveillance range is shown in FIG. 2 by reduced perimeter 261. Shortening the surveillance range 261 may be dangerous for aircraft flying at high speeds, as warning time and time to act on a resolution advisory may be significantly reduced.
Presently, under the requirements of the FAA and various other airworthiness authorities in several countries, only one or few aircraft in a formation is allowed to have an actively interrogating TCAS (referred to herein as “active TCAS”.) If all the members in a formation are not interrogating, significant safety problems can arise. That is, the non-interrogating formation members will not be aware of potential collision threats between themselves and oncoming, non-formation aircraft because their respective TCAS is switched off. The non-interrogating members of the formation will also have no warning by their respective TCAS of potential collisions with other formation members.